1. Field of the Invention
The present invention relates generally to the field of mechanical equipment suitable for forming bells or sockets in one end of length of plastic pipe to permit joining adjacent lengths of plastic pipe in end to end juxtaposition by inserting the unbelled or spigot end of one length of pipe into the bell or socket which is formed in the proximate end of the next adjacent lengths of pipe.
2. Description of the Prior Art
In prior art types of pipe belling equipment, it has been known to position a shaped mandrel of predetermined configuration and size within a belling machine and to serially advance discrete lengths of plastic pipe, each having one end preheated, into axial alignment with the belling mandrel. Suitable operating mechanisms have been developed and have been utilized to either axially move the pipe onto and over the mandrel, or alternately, to axially move the mandrel into the preheated end of the pipe for pipe end shaping into the desired bell or hub-shaped configuration. In certain designs, the prior pipe belling operations have been conducted within a suitable environmental chamber where suction forces or pressure forces could be introduced into the chamber to surround the pipe and thereby pull or press the preheated, softened pipe walls uniformly against the outer periphery of the mandrel.
In accordance with more recent popular practice, it has become increasingly important to provide an internal groove in the bell configuration when the bell or hub shape was formed to provide a convenient annular internal groove for seating therein a resilient gasket in a manner to facilitate the making up of a leak proof junction between adjacent lengths of pipe in a piping system. Mandrel constructions have been developed by prior workers in the art which are capable of forming a circular, internal groove in the hub configuration for receipt of a gasket therein when the bell or hub of the plastic pipe was being formed.
However, so far as is known to the present applicants, despite all of the efforts of the many prior workers in the field to improve the joining procedures and constructions when utilizing plastic pipes, the proper and permanent seating of a sealing gasket in the junction between the spigot end of one length of pipe and the hub end of the next adjacent length of pipe, either with or without a peripheral internal groove formed in the hub, usually present field problems, and this is especially true in large diameter work. Particularly, in many designs the gaskets have been subject to displacement or "fishmouthing" when the joint was being formed. In other instances when the system was operated under pressure, gasket blow out could frequently occur in the event of pressure surges in the line.
In the plastic pipe industry, it is the common practice to provide a rubber or other elastomeric material gasket to form the seal between adjacent pipe sections or at the junction between a pipe and a fitting. Such gaskets have been configured and designed to facilitate the making up of the pipe joint in the shortest possible period of time in a manner to prevent leakage at the joint upon completion of the system. As the design operating pressures in a piping system increase, the difficulties encountered in providing an efficient and reliable seal have also increased.
In efforts to solve the sealing problems at pipe joints which are inherent in pressure or vacuum piping systems, prior workers in the art have developed gaskets of the type including one or more radially inwardly extending lips to press against the spigot end of the connected section of pipe. Additionally, other workers have provided gaskets having an internal peripheral groove with in which is seated a locking ring of relatively hard plastic material to prevent movement of the seal as the joint is made up and to discourage gasket blow out when surges in system pressure occurred. In other efforts to prevent unseating of the gasket when the pipe joint was made up or to aid in preventing gasket blow out under severe pressure conditions of use, prior workers have also attempted to develop means to lock the gasket within an annular groove which could be formed in the plastic material of the bell during the bell forming operation.
In U.S. Pat. No. 4,343,480, entitled "Pipe Bell and Gasket", which patent is owned by the assignee of the present application, there is disclosed a bell and spigot connection for plastic pipes. The patented design employs a gasket groove in the hub and a gasket seated within groove and having a pair of longitudinally spaced, radially inwardly extending lips. The gasket may be utilized either with or without a hard plastic retaining ring intermediate the lips. In the patented configuration, the exterior or entrance end of the hub was radially inwardly bent after the gasket was seated in the peripheral, inwardly open groove which was formed in the hub to thereby retain the gasket within the groove in a manner to prevent gasket blow out upon surges in system pressure.
It has been found that the prior art hub and gasket constructions were particularly difficult to form to precise tolerances and this was especially true in large diameter work wherein the very thickness of the large diameter pipe sidewall constructions resisted efforts to satisfactorily develop the machinery and techniques necessary to form the configurations of bells and gasket grooves within the tolerances required to assure proper gasket performance both when the joint was made up and after the piping system was pressurized. The need remains to provide a functional, reliable gasket seating arrangement for the bells of plastic pipes.
As used herein, the term "large diameter" is defined to mean plastic pipes having an inside diameter of ten inches and larger.